The present invention relates to a hydraulic power steering apparatus comprising an oil pressure regulating valve that is provided inside a housing having a cylindrical shape for supporting a steering shaft and that supplies and discharges hydraulic fluid to and from a power cylinder for steering assist.
A hydraulic power steering apparatus assists steering by means of oil pressure generated by an oil hydraulic cylinder (power cylinder) of double acting type arranged in a steering mechanism, thereby reducing labor work of a driver so as to provide comfortable feeling in steering. In such a hydraulic power steering apparatus, an oil pressure regulating valve for performing supply and discharge operation for oil pressure in response to the direction and the magnitude of a steering torque applied to a steering member such as a steering wheel is arranged between each of two cylinder chambers of a power cylinder and each of an oil hydraulic pump serving as an oil pressure generation source and an oil tank serving as a drain oil destination (see, for example, Japanese Patent Application Laid-Open No. H10-324259(1998)).
An oil pressure regulating valve widely used is of rotary type which utilizes the rotation of a steering shaft for transmitting an operation of a steering member to a steering mechanism. Specifically, the steering shaft is divided into an input shaft connected to the steering member and an output shaft connected to the steering mechanism, while these shafts are coupled coaxially via a torsion bar. Then, into a valve body having a cylindrical shape engaged at one coupling end, a valve spool formed integrally at the other coupling end is fitted in a manner permitting relative rotation. Thus, when a steering torque is applied to the steering shaft, relative angular displacement arises between the valve body and the valve spool, with causing torsion in the torsion bar.
In each of the fitting circumferences of the valve body and the valve spool (that is, the inner periphery of the former and the outer periphery of the latter), a plurality of oil grooves that extend in the axial direction are aligned in the circumferential direction. In each of the valve body and the valve spool, the oil grooves are arranged in a staggered manner in the circumferential direction. When these components are assembled, a plurality of throttle sections for changing throttle areas depending on the relative angular displacement are formed between adjacent oil grooves. Each of these oil grooves is provided with: an oil supply groove and an oil drain groove connected respectively to the oil hydraulic power source and the drain oil destination; and a pair of oil feed grooves each of which is located in between with each oil drain groove on each side of the oil supply groove, and each of which is separately connected to both cylinder chambers of the power cylinder serving as the feed oil destination.
In a neutral state that no steering torque is applied to the steering member so that no torsion is caused in the torsion bar, the throttle sections aligned on the fitting circumferences of the valve body and the valve spool have the same throttle area with each other. At that time, the pressure oil supplied from the oil pressure generation source to the oil supply grooves is distributed and introduced equally to the oil feed grooves adjacent on both sides, then introduced into the oil drain grooves adjacent on the other side, and then discharged to the drain oil destination. At that time, no pressure difference arises between the cylinder chambers in communication with the oil feed grooves. Thus, the power cylinder generates no force.
On the other hand, when a steering torque is applied to the steering member, relative angular displacement arises between the input shaft and the output shaft, with causing torsion in the torsion bar. This causes a change in the throttle areas of the plurality of throttle sections aligned on the fitting circumference. At that time, the high-pressure hydraulic fluid supplied from the oil pressure generation source to the oil supply groove is introduced into one oil feed groove via a throttle section that has an increased throttle area. Thus, a pressure difference arises between one cylinder chamber in communication with one oil feed groove and the other cylinder chamber in communication with the other oil feed groove. In response to the pressure difference, the oil pressure generated by the power cylinder is applied to the steering mechanism, so that steering is assisted.